But it's been "on" before, right?

I would love for an IP attorney to weigh in on this. If the trademark is approved by the USPTO, then Nintendo has a legal obligation to pursue every use of the phrase that would infringe upon the mark in such as way that causes irreparabe damage to Nintendo. Is that wise legal strategy, to embroil the company in additional litigation, perhaps unnecessarily?

Plus this is an example of a catchphrase in the mainstream, and a positive one at that as far as the brand is considered. Nintendo should be so lucky. So would the USPTO reject the mark based on that?

Limitations on trademarks

Let's not forget, a trademark only comes with the exclusive right to use it in a particular trade. This isn't like Nintendo would get the right to stop anyone else from using the expression "it's on like Donkey Kong"--only to stop anyone from using it in such a way that it might be misleading or confusing. I feel like Nintendo has a legitimate interest in preventing, say, Sony or Microsoft from using "it's on like Donkey Kong" to market their competing video game systems.

Now that's not to say that trademarks aren't often abused by rights-holders making unjustified threats, but I don't think registering and using a mark like this inherently abusive.

Re:

If your penis tosses barrels, then yes, there would be some distinct similarity issues. I strongly suggest adopting some more unique, alternative objects to toss with your penis. Perhaps ball point pens, or dry cat food, maybe paper cups with pictures drawn on them.

Dino: Nothing in the law requires a trademark holder to file additional applications for registration covering phrases that use a mark it owns. It, of course, can register such variations for strategic reasons--but law doesn't obligate it to do so. Indeed, that's, in part, what the likelihood of confusion legal test for trademark infringement is designed to prevent. The law does, however, require you to "protect" the marks you own. As for trademarking a popular phrase . . . "Just do it." I don't see anything special about this.